Continuous production of ceramic oxide superconductors, for example of the yttrium, barium and copper oxide type (YBCO), is assured by means of a device consisting of the combination in cooperation relationship of: a continuous movable surface, means for feeding a liquid layer containing the desired proportion of metal ions on to said continuous surface, means for evaporating the liquid component of said liquid layer and for hightemperature roasting of the remaining solid residue in an oxidizing environment, and means for removing from said surface the product obtained by said roasting.
The present invention concerns a device for the continuous production of ceramic superconductors. More precisely it concerns the production of ceramic oxide superconductors by means of a device for ensuring a thin-layer pyrolysis of a solution containing ions of the metals which will be part of the desired final ceramic.
Until a few years ago, the use of superconductivity was limited by the cost of the technique, owing to the very low temperatures needed for the phenomenon to occur, typically at around that of liquid helium.
However, practical interest in superconductivity has been greatly aroused by the recent discovery of new classes of materials, especially ceramic oxides, that manifest this property at much higher temperatures which are more economical to attain and maintain, typically around that of liquid nitrogen. However, this interest has been somewhat restrained by the poor availability of ceramic oxide superconductors, essentially due to the unavailability of methods and devices capable of ensuring qualitatively and quantitatively satisfactory production of said materials.
In fact, almost all ceramic oxide superconductor production techniques are excessively costly (as, for instance, the supergrinding of the oxides composing the desired ceramic; the coprecipitation, described inter alia in J. Phys; 30, (1988) L251; the sol-gel method, described for example in ACS Symp. Ser. 351 (1987) ch. 10; or the citrates method, described in J. Phys. D21 (1988) 226), their productivity is not commercially acceptable, their realization can be complex and with unsatisfactory results. Known methods for the continuous production of ceramic oxide superconductors are almost all based on variations of an old process for the production of oxides (U.S. Pat. No. 3,189,550), consisting in atomizing a solution containing the desired mixture of metallic ions in a furnace and then collecting the ensuing powders. A variation consists in directing the jet of ceramic powders thus formed on to a surface to be coated (examples of these techniques are reported in Jap. J. of Appl. Phys. vol. 27, No. 6, pages L1086 to L1088 and pages 1092 to 1093, as well as is vol. 29, No. 1 pages L33 to L35).
The main drawbacks with these continuous techniques concern:
low productivity: the powders produced are normally very fine (around one micron or less) and are very difficult to collect, since--as known--the efficiency of electric filters is less than 70% for such fine particles;
the morphology of the particles, which are generally hollow; hence the bodies produced with them have a low green density (typically less than 60%) making it difficult to ensure that the final dimensions are as desired (see french Patent Appl. No. 2628415)
impossibility of controlling the thermal treatment of the particles because they are produced in a highly turbulent fluid environment.
It is thus evident that present techniques are still not capable of furnishing production systems capable of satisfying the big demand for ceramic oxide superconductor powders which has recently emerged.
The present invention overcomes the difficulties mentioned by supplying a simple, easily-controllable device for the continuous production of ceramic oxide superconductor powders of excellent quality and capable of ensuing compacted products of the maximum possible density right from the green state.
According to the present invention, a device for the continuous production of superconductors, starting from a solution comprising a liquid component in which are dissolved metal ions in the desired proportions, is formed by the combination in cooperation relationship of:
a continuous movable surface,
means for feeding a liquid layer of said solution on to said continuous surface,
means for evaporating the liquid component from said liquid layer and for treating the residual metal salt in a high-temperature oxidizing environment to give ceramic oxides,
means for removing the ensuing ceramic oxides from said continuous surface;
Preferably, said continuous surface is one of the surfaces of an endless metal belt; in order to avoid pollution of the desired product, said surface preferably has a coating that is resistant to high temperatures, preferably oxide-based and inert to said solution containing the metal ions.
A component considered satisfactory for this purpose is magnesium oxide, which can be associated with other oxides to form compounds such as fosterite.
Said surface is made to pass continuously through said means capable of evaporating the liquid component of said liquid layer and oxidizing the remaining solid residue at high temperature, said means consisting preferably of a tubular furnace divided lengthwise into a plurality of zones, each one being provided with its own means for temperature control and monitoring. Said tubular furnace is advantageously heated electrically and can be equipped with means for controlling and monitoring a possible flow of gas through it.
The means for feeding on to said continuous surface a liquid layer containing the ions of metals which must form the oxide superconductors, preferably consist of a tank for a liquid, complete with means for control and monitoring liquid flow towards said continuous surface; to ensure good continuous uniform treatment across the whole width of said surface, the device as per the invention is advantageously equipped with means for ensuring that said layer is of uniform thickness for instance consisting of at least one skimmer blade. Said continuous surface can also be equipped with lengthwise side-strips to confine said liquid and at least one vertically-adjustable blade set at a distance from said surface such as to determine the desired thickness of said liquid layer; in that case, said blade, or at least the last of these blades, is set at the entrance to said furnace, so that said liquid can be rapidly vaporized before uncontrolled thickness variations occur.
Said means for removing the products of the oxidation which occurs in the furnace from said continuous surface, consist preferably of a scraper bar, set skew to the direction of travel of said surface, which just touches the surface itself. Provision is made for means to collect the product thus obtained; this takes the form of a collection bin positioned beneath said surface at the location of said bar.